Ill 



PHYLUM AND CLASS PORIFERA 



121 



classes. In the Calcarea they consist of calcareous spicules, usually 

 tri-radiate in form. Each of these spicules is developed from special 

 cells the scleroblasts (Fig. 90). In the remaining groups of Sponges 

 the skeleton either consists of spongin fibres alone (Fig. 91, A), 

 or of siliceous spicules alone, or of a combination of spongin fibres 

 with siliceous spicules (B) : in some Demospongia (the Myxospongia) 



A.Eus^ongia 



B.Pachychalina 



FIG. 91. Microscopic structure of the skeleton in various sponges. A, Euspongia, network 

 of spongin fibres ; B, Pachychalina, spongin strengthened by siliceous spicules ; C, 

 Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules 

 of other sponges, <fec. (After Vosmaer.) 



skeletal parts are altogether absent. Spongin is a substance allied 

 to silk in chemical composition and contains a large amount of 

 iodine : the fibres are exceedingly fine threads, consisting of a soft 

 granular core and an outer tube of concentric layers of spongin. 

 These threads branch and anastomose, or are woven and felted 



